1. Field of the Invention
The present invention relates in general to a method and apparatus for determining the concentration of water in an oil water fluid mixture. More particularly, the present invention relates to a method and apparatus for determining the concentration of water within an oil water mixture based on the identification of a specific frequency transmitted to the fluid mixture that results in the lowest frequency signal after reacting with the fluid mixture.
2. Description of the Related Art
Knowledge of accurate water content in a petroleum fluid mixture is important for oilfield reservoir management, royalty allocation, buying and selling, corrosion management, refining, chemical processing, and aviation safety.
Current oil water measurement devices typically measure the ratio of fluids in an oil water fluid sample using radio frequency (RE) or microwave energy to determine the capacitance or permittivity of a fluid sample consisting of oil and water. Because permittivity is the measurement of a fluid's ability to resist an electronic field, the power required to maintain a specific frequency within that fluid is a function of permittivity.
In one embodiment of the currently available microwave systems, the oil water measurement device is emerged in a fluid emulsion of oil and water. Within the oil water measurement device an oscillator is connected to a single antenna. The antenna transmits one or more specific frequencies to the fluid emulsion and receives the load change in the current consumption of the oscillator. The current consumption of the oscillator is dependent on the composition of the mixture since various components have different dielectric constants, which are proportionally related to the overall impedance of the mixture.
The complex permittivity of many materials changes with the frequency used for the permittivity measurement. Thus, as the oscillator frequency is changed, the complex permittivity also changes and the resulting system of mathematical equations used to describe and solve for the component concentrations become increasingly non-linear. If, however, the permittivity measurements can be made at accurate and repeatable frequencies, the fluid system components could be determined hour simple linear equations.
Some of the newer microwave apparatus use multiple oscillators or voltage-controlled-oscillators (VCOs) to measure water concentrations in oil water mixtures at one or more specific frequencies. As with any electronics, these oscillators are subject to drift due to the temperature of the ambient surroundings or from self-heating and aging of the components. It is difficult, or impossible, to separate drifts in the oscillator from actual impedance changes in the fluid medium and as explained previously, non-linear dielectric constants tend to magnify the measurement errors.
A few of the systems include a reference oscillator calibrated to provide a specific frequency for a known impedance, but the reference oscillator is subject to the same thermal and aging errors. In fact, component aging and thermal effects might have offsetting effects and move the reference frequency in the opposite direction from the measurement frequency. Thus, the reference and measurement oscillators require frequent calibration and recalibration.
The device described in U.S. Pat. No. 7,135,870 measures the power consumed by the crystal oscillating loop and uses that measurement to determine the permittivity of the fluid which is related to the water content in the fluid. Signals, such as radio frequency or microwave, are sequentially transmitted at multiple, known, constant frequencies to and reflected from both a reference sensor and a measurement sensor. Permittivities of the individual components are determined from these transmitted and reflected signals and information about the concentration of the components of the fluid mixture is calculated.
There is an existing need for a simple automated device for accurately measuring water content in petroleum fluids containing low concentrations of water.